1. Field of the Invention
The present invention relates to an electric power system interconnection device for providing uninterrupted electric power to a load in a stable fashion even in the event of a failure of a primary power supply by establishing coordinated interconnection of the primary power supply and a secondary power supply via a power converter.
2. Description of the Background Art
FIG. 13 is a block diagram of an uninterruptible power supply unit, a conventionally known example of an electric power system interconnection device, disclosed in Japanese Examined Patent Publication No. 5-76256. In this Figure, designated by the numeral 1 is an alternating current (AC) power supply, designated by the numeral 2 is a mechanical switch which connects or disconnects the AC power supply 1 to or from a load 3, designated by the numeral 4 is a power converter which is connected to a line connecting the switch 2 and the load 3 and converts direct current (DC) to AC, and AC to DC, designated by the numeral 5 is a storage battery, designated by the numeral 6 is a charging control circuit which controls charging of the battery 5 (or the DC power supply) from the AC power supply 1 when it is under normal operating conditions, designated by the numeral 7 is a constant voltage control circuit which controls the power converter 4 such that it outputs a specified constant voltage, designated by the numeral 8 is a selector which selects an output signal of the charging control circuit 6 or an output signal of the constant voltage control circuit 7, designated by the numeral 9 is a pulse-width-modulation (PWM) gate drive circuit which generates an operation command to be given to the power converter 4 based on the output signal of either the charging control circuit 6 or the constant voltage control circuit 7 selected by the selector 8, and designated by the numeral 10 is an interrupt control circuit which outputs an interrupt signal to open the switch 2 upon detecting an abnormal condition of the AC power supply 1.
Operation of the uninterruptible power supply unit is now described below.
When the AC power supply 1 is under normal operating conditions, the switch 2 is closed allowing the AC power supply 1 to deliver electric power to the load 3. Under these conditions, the selector 8 selects the output signal of the charging control circuit 6 so that the power converter 4 is controlled by the output signal of the charging control circuit 6 via the PWM gate drive circuit 9 and charges the battery 5.
When an abnormal condition occurs in the AC power supply 1, the interrupt control circuit 10 opens the switch 2, and the selector 8 selects the output signal of the constant voltage control circuit 7 upon receiving an interrupt signal produced by the interrupt control circuit 10, whereby the constant voltage control circuit 7 controls the power converter 4 via the PWM gate drive circuit 9 such that the power converter 4 outputs a constant voltage. It is therefore possible to convert electric energy fed from the battery 5 into stable AC power by the power converter 4 and deliver it to the load 3 even in the event of a failure of the AC power supply 1.
When the conventional electric power system interconnection device thus constructed detects an abnormal condition of the AC power supply 1, the switch 2 is opened and at the same time the power converter 4 is switched to output the constant voltage, whereby the electric energy of the battery 5 is converted and supplied to the load 3. The switch 2 used in this kind of electric power system interconnection device is a mechanical switch which generally offers such advantages that it is available at low cost, the amount of heat generation is small and it has high switching surge and overcurrent withstand capabilities. High-speed mechanical switches developed in recent years operate at a speed of about 1 millisecond. However, although the mechanical switch 2 of this kind is opened with a mechanical action of such a high speed, the switch 2 is actually opened electrically only when an arc current flowing through the switch 2 becomes zero. In a case where the AC power supply 1 is commercial AC power supply, this delay time, from an off command (interrupt signal) to electrical isolation of the AC power supply 1, could reach about 10 milliseconds at worst which is 0.5 cycle of the AC power supply waveform (in the case of 60 Hz power frequency) even if the switch 2 operates at an ideally high speed. Even when the switch 2 is a non-mechanical semiconductor switch like a thyristor which does not have a self-turn-off capability, the switch 2 is not electrically opened until the current becomes zero.
For reasons stated above, even when the constant voltage control circuit 7 begins to control the power converter 4 to have it output a constant voltage following the off command for opening the switch 2, the power converter 4 remains connected to the AC power supply 1, in which an abnormal condition has occurred, until the switch 2 is completely opened electrically. Therefore, the power converter 4 can not output the constant voltage in a fully reliable fashion. This produces such a problem that the power converter 4 can not output a correct voltage to the load 3 when a short circuit has occurred in the AC power supply 1, for instance, causing the load 3 to become inoperable and abnormally stop.
Accordingly, the present invention is intended to provide a solution to the aforementioned problems of the prior art. Specifically, it is an object of the invention to provide an electric power system interconnection device which can quickly suppress an arc current flowing across contacts of a mechanical switch when the switch connecting a primary power supply and a load is opened, making it possible to electrically isolate the load from the primary power supply at a high speed in the event of a failure of the primary power supply and then supply stable electric power to the load.
In a first principal form of the invention, an electric power system interconnection device for providing uninterrupted electric power to a load by establishing coordinated interconnection of a primary power supply and a secondary power supply, in which the primary power supply is connected to the load via a switch and the secondary power supply is connected to a load side of the switch via a power converter includes an interrupt control circuit, an interrupt control circuit, a switch current interrupter for accelerating interruption of a current flowing through the switch, and a constant voltage controller for maintaining a constant voltage to be applied to the load. The interrupt control circuit outputs an interrupt control signal for opening the switch upon detecting an abnormal voltage fed from the primary power supply. The switch current interrupter for accelerating interruption of a current flowing through the switch accelerates interruption of the current flowing through the switch by controlling an output current of the power converter by a specific output current command using the interrupt control signal as an input. The constant voltage controller for maintaining a constant voltage to be applied to the load maintains the voltage applied to the load at a fixed level by controlling the power converter to output a specific constant voltage upon detecting the interruption of the current flowing through the switch.
The electric power system interconnection device of this form can quickly suppress an arc current flowing through the switch when the switch is opened so that the primary power supply is electrically disconnected from the load at a high speed in the event of a failure of the primary power supply and the electric power system interconnection device can supply stable electric power to the load.
In a second principal form of the invention, an electric power system interconnection device for providing uninterrupted electric power to a load by establishing coordinated interconnection of a primary power supply and a secondary power supply, in which the primary power supply is connected to the load via a switch and the secondary power supply is connected to a load side of the switch via a power converter includes an interrupt control circuit which outputs an interrupt control signal for opening the switch upon detecting an abnormal voltage fed from the primary power supply. This electric power system interconnection device controllably operates the power converter such that its output voltage becomes a specific constant voltage by producing an output current command based on the interrupt control signal, causing the power converter to feed a current flowing in a reverse direction to the switch to instantly interrupt a current flowing through the switch. The voltage applied to the load is then maintained at a fixed level.
The electric power system interconnection device of this form can quickly suppress an arc current flowing through the switch when the switch is opened so that the primary power supply is electrically disconnected from the load at a high speed in the event of a failure of the primary power supply and the electric power system interconnection device can supply stable electric power to the load with a simple and low-cost circuit configuration.
These and other objects, features and advantages of the invention will become more apparent from the following detailed description and the accompanying drawings.